Based on previous work, it is now evident that at least one species of teleost fish uses a form of echolocation to detect the presence of nearby obstacles. The sea catfish, Arius felis, produces low frequency sound pulses that function in social communication. In addition, the reflections and reverberations of these sounds assist in detecting objects in the immediate area. Experimental evidence shows that this function may be enhanced by the direction receiving and projecting characteristics of the swim bladder of this species. Electrophysiological evidence supports the role of the lateral line system in directional near-field detection. The present proposal is to investigate and measure the range and accuracy of the echolating capacities in this species; to evaluate the role of the lateral line behaviorally as well as neurophysiologically; to test the directional acoustics of the swim bladder under free-field (open water) conditions. In a long term segment of this project, newly hatched and juvenile sea catfish will be tested for sound production, and the development of sonic behavior will be correlated with the morphological development of the sonic and hearing mechanisms. The echolating mechanisms in this species are judged to be primitive and simple, as compared to those of bats and dolphins, and, as such, can provide significant information for the study of other simple echolating systems, such as those that can be developed in humans.